Treatment of gelatin



Patented Feb. 8, .1944 i N. Y., assignors to Eastman Kodak Company,Rochester, N. Y., a corporation of New Jersey No Drawing. ApplicationSeptember 11', 1040, Se-

rial No. 356,354. In Great Britain January 5,

' 3Clairns. (01.2604418) This invention relates to treatment of gelatin.It is known that gelatin finds many uses in technology and especially inthe preparation of photographic products. Gelatin, being a naturalproduct, is liable to considerable variation in some of itsconstituents, from batch to batch, even though the batches are of thesame type of gelatin. These variations can, and frequently do, causeconsiderable variation in the behavior of the gelatin in practice.

In the preparation of gelatin to be used for photographic products, itis necessary to give the gelatin a special treatment or purification inorder to remove substantially all of certain of the mineral matteroccurring naturally in the gelatin. Particularly Where the gelatin is tobe used as a medium in which acid dyes, especially sulfonated dyes, aredispersed, it is essential to substantially remove mineral matter, suchas calcium, from the gelatin, in order to avoid crystallization orprecipitation of the dyes in the gelatin. In removing mineral matter,such as calcium, from gelatin intended for use in the preparation ofphotographic products, it is essential to avoid substantially, anyappreciable disturbance of the properties of the gelatin which make it auseful substance in the preparation of photographic products, such asphotographic emulsions. Thus, it is essential to avoid substantiallychanging the pH of the gelatin and to avoid substantial removal ofcertain constituents which affect the sensitivity of the light-sensitivematerials which may be introduced into the emulsion.

It is known that calcium can be substantially removed from gelatin bywashing the gelatin with aqueous solutions of common salt (about 10% byweight, in strength) which solutions contain hydrochloric acid. Thismethod has the great disadvantage of changing the pH of the gelatinconsiderably. Because of this change, it is proposed in this knownprocess, to follow the Washing with the acidulated salt solution, with awashing with a neutral 1% salt solution, in an attempt to re adjust thepH of the gelatin. The calcium, however, is removed during the washingwith the acidulated salt solution.

We have now found that calcium can be substantially removed from gelatinby treating the gelatin with a neutral solution of an alkali metal orammonium salt. By avoiding the acid employed in the prior art processes,we avoid any appreciable change in the pH of the gelatin. Moreover, ournew process has the further advantage of not causing a great deal ofswelling of the gelatin, when we employ neutral solutions of alkalimetal or ammonium sulfates. It is altogethersurprising'that sulfates ofalkali metals or ammonium readily remove calcium from gelatini sincecalcium sulfate has a-verylow solubility in water.

Our new process is not to be confused with processes where gelatin isdecalcified by treatment with alkali metal or ammonium metaphosphates.In the metaphosphate processes, the calcium is removed from the gelatinby virtue of the formation of alkali metal (or ammonium) calciummetaphosphate complexes, whereas in our new process there is an actualinterchange of cations, the alkali metal or ammonium cation from theneutral solution actually replacing the calcium in the gelatin, therebyproducing an alkali metal or ammonium gelatin complex,

It is, accordingly, an object of our invention to provide an improvedprocess for purifying gelatin. Other objects will become apparenthereinafter.

In accordance with our invention, we treat gelatin with a neutralsolution of an alkali metal (sodium, potassium, lithium, etc.) orammonium salt. chlorides, nitrates, perchlorates and sulfates can beused, although nitrates produce considerable swelling of the gelatin.The sulfates have the great advantage of suppressing the swelling of thegelatin. Moreover, neutral sulfates have no appreciable action on aphotographic gelatino-silver-halide emulsion, so that a shreddedemulsion can be decalcified by treatment with a solution of sodiumsulfate as an integral step in the emulsion making technique, i. e., atthe convention emulsion washing step.

The following example will serve to illustrate the manner of practicingour invention. This example is not intended to limit our invention.

' Example Dried leaf gelatin containing 0.4% calcium was immersedfor 30minutes in a 10% solution of sodium sulfate (NazSOr) It was then washedin a few changes of distilled water (hard waters containing calciumshould not be used for washafter acidification of the gelatin, isprobably present as calcium phosphate.

The very small percentage The present method of deoalcifying gelatin isof particular value, when for any reason, it is desired to remove thecalcium from a manufactured gelatin, especially as it is not necessaryto dissolve the gelatin, as it is in the zeolite process, described inthe copending application of Edwin E. Jelley, Serial No. 296,320, filedSeptember 23, 1939, now Patent No. 2,273,577,

Gelatin prepared in accordance with our invention is of particularlyvalue in the preparation of photographic gelatino-silver-halideemulsions, and in the preparation of light filters, where acid dyes, e.g., chrysophenine, are introduced into the gelatin according to thevarious needs of photography.

What we claim as our invention and desire to be secured by LettersPatent of the United States 1. A process for decalcifying gelatincomprising replacing the calcium in a calcium-containing-gelatin with acation selected from the group consisting of alkali metal and ammoniumcations, by treatingwithout dissolving the calcium-containing-gelatinwith a neutral aqueous solution of a salt selected from the groupconsisting of alkali metal and ammonium normal sulfates, removing thegelatin from the aqueous solution, and then Washing the gelatin in watercontainin no more calcium than the gelatin.

2. A process for decalcifying gelatin comprising replacing the calciumin a calcium-containing-gelatin with an alkali metal ion, by treatingWithout dissolving the calcium-containing-gelatin with a neutralsolution of an alkali metal sulfate, removing the gelatin from theaqueous solution, and then Washing the gelatin in water containing nomore calcium than the gelatin.

3. A process for decalcifying gelatin comprising replacing the calciumin a calcium-containing-gelatin with a sodium ion, by treating withoutdissolving the calcium-containing-gelatin with a neutral solution ofsodium sulfate, removing the gelatin from the aqueous solution, and thenwashing the gelatin in water containing no more calcium than thegelatin.

EDWIN E. JELLEY. WALTER J. WE'YERTS.

